I eventually installed manually linux-4.8.6-1 but I am afraid I broke something with updates / something is wrong.
> sudo pacman -S linux !6272 warning: downgrading package linux (4.8.6-1 => 4.8.4-1)
> yaourt linux 20 core/linux 4.8.4-1 (base) [installed: 4.8.6-1] The Linux kernel and modules
> uname -a Linux home 4.8.6-1-ARCH #1 SMP PREEMPT Mon Oct 31 18:51:30 CET 2016 x86_64 GNU/Linux
sudo pacman -Syyu < wants to downgrade
Obviously correct version (4.8.6-1) is present in repositories: https://www.archlinux.org/packages/core/x86_64/linux/
I don’t think I changed anything but yet here we are :/ I am afraid my kernel might not update automatically when a new version will be available, do you have any idea what to do?